Described below is a method of dispersion of mobile terminals in a communication system, in particular for second and third generation mobile phone systems, such as Universal Mobile Telecommunications System Terrestrial Radio Access Network (UTRAN), offering multicast broadcast multimedia services' (MBMS) or other broadcast services.
Broadcast services, such as MBMS which is standardized in radio access network (RAN), are becoming more and more important for mobile communication networks. Studies of MBMS have shown that MBMS may require up to 30% to 40% of NodeB available power, depending on the scenario, which means most operators may only provide MBMS on one wideband code division multiple access (WCDMA) carrier. Limiting to a single carrier is also beneficial for mobility purposes and for combining gains at cell edges. As the traffic in the network increases, operators may have to use more than one of their frequencies. In particular, a second carrier would be required for micro and Pico NodeBs to cover hot spot areas with high traffic volumes.
Thus, frequency layer convergence (FLC) for MBMS is applied so that an MBMS notification is sent in all cells on all frequencies indicating a final service providing layer. All user equipments (UEs) interested in receiving the notified service via MBMS then re-select to this layer, which is brought about by applying certain offset values. MBMS operation may be performed on a single frequency layer (in a multi-frequency UTRAN network) by indicating to the UE the layer in which the MBMS data is being sent and an offset which is applied to the TIE cell reselection parameters to ensure that the UE moves to the preferred frequency when it wants to receive the MBMS service.
However, having reached the single layer and duly received the MBMS service, termination of the MBMS service means that the offset is no longer applied and the UEs return to normal operation. However, the UEs are prone to remain camped in the new macro cell on the MBMS frequency, since existing cell reselection procedures in the UTRAN are defined such that a TIE may prefer to stay on in a current cell or existing frequency, even if it is not the best cell or frequency for that UE, so as to avoid continuous reselection for UEs on a border between cells. This concentration of UEs in a particular cell at the end of the service is undesirable because any additional movement of the UE between cells in the undesirable frequency will result in uplink signalling which causes more interference than if the UE had been on a more appropriate frequency. This increase in interference may result in instability in the system, and a reduction in the cell radius.
Proposals have been made to deal with this problem by imposing a random distribution of the UEs in the cell at the end of the MBMS session, to different frequencies and cells. However, this leads to subsequent reselections according to normal cell reselection rules, as the cells move to their favored cells, so the increased number of cell reselections gives rise to an increase in uplink signalling, if the UE is in cell forward access channel (Cell_FACH) or cell paging channel (CelLPCH) state.
The end of the service is detected by the UE reading MCCH and receiving the session stop information. Alternatively the UE can read the MCCH and detect that the session which it was previously receiving on MTCH is no longer described in the information on MCCH, indicating that the session is no longer being sent on MTCH.
After a service is complete, the offset is no longer applied, and the UE (under current assumptions and specification) returns to normal operation while still camped in the new cell. Existing cell reselection procedures in the UTRAN are defined such that a UE may prefer to stay on the existing cell even if it is not the best cell in order to avoid continuous reselections for UEs on a border between cells. Hence the existing procedures are not optimum for dispersing UEs after an MBMS session has completed.